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Topic: Shotgun mic test in a poor room (Read 4058 times)

I had posted this topic on GS and Voltronic suggested I also post it here.

Something I had been wondering is whether a shotgun mic could be used in a large room. I had read that shotgun mics are pretty much for outdoor use only because the reflections you get indoors will cause nasty effects with the interference tube. However I seemed to remember reading somewhere about Tony Faulkner at some point having used shotgun mics indoors. So I decided to try this out.

Because connected to the above is the fact that most of my recording is done in a room with less than ideal acoustics. In addition it has a loud ventilation system that actually does have an "off" switch except it is currently broken (and always on). Also the compressor to the skating rink in the same building is behind one of the walls. So my thinking was that IF the room is big enough to avoid the effects from the interference tube, it might just work in cutting down the acoustics a bit.

Recently on eBay I found someone selling several Sennheiser MKH805's. This is a mid to late 1960's long (21") shotgun mic. T-power. I bought two (thinking that if I liked it, I probably wouldn't be able to find a second one), and bought a T-power adapter and a Rycote INV-BH shockmount (which turn out to be somewhat difficult to find around here).

The other day I went to the afore mentioned room (the place where my community band rehearses; I have a key to the place) in the afternoon so I would have plenty of time by myself to do some test recordings.

When I record a concert I usually have to be in the far corner of the room. The conductor doesn't really like a large stand behind him, and moreover there tends to be a lot of foot traffic there. So I need to be behind the audience, about 9 meters away. Another reason why a shotgun might be useful.

The test setup was a pair of Advanced Audio CM1084's with super cardioid capsules, 60cm spacing and angled 40 degrees. The Sennheiser MKH805 in the middle, straight ahead and for reference a CM1084 with an omni capsule.

Attached pics of the room and the test setup. The gray chair is where I was standing for the recordings.

Due to size limitations of attachments of 500KB on TS I had to cut the recordings down quite a bit. Here is the omni, the left hand supercardioid (the right hand one sounds pretty much identical to me, so I don't think it makes much difference) and the MKH805. I adjusted these to the same LUFS with no other processing.

As for the noise of the ventilation system, the omni is worst. The MKH805 is in the middle and the supercardioid is best. However, with respect to room acoustics, I personally like the MKH805.

As a result of the discussion on GS and an excellent suggestion by Voltronic I cleaned the files up with Izotope RX4. Results to follow in the next post.

As I mentioned in the previous post, I cleaned the files up in RX4 using Voltronic's method (see: https://www.gearslutz.com/board/11338967-post9.html). The makes a huge difference in the noise from the ventilation system and makes it easier to hear the acoustics of the place. The MKH805 has more audible noise left, but cutting it down further makes it sound strange.

To my ears and on my speakers, I can hear more room with the supercardioids. The MKH805 sounds drier to me, which was one of the objectives of the test.

I have decided to buy the components I need to activate that second MKH805 (a second T-power adapter and Rycote INV-BH) and then redo the test in stereo.

I don't think the objections I've seen expressed here are against indoor use, but rather use in a stereo pair where off-axis response is desired as part of the stereo signal, because the off-axis response varies greatly by angle and frequency. That was or is a somewhat popular method for outdoor recordings where some type of stereo image is being attempted that still rejects the audience, but likely will sound like a phasey mess.

That might very well be the case, sounds entirely plausible. I enjoy doing experiments to test things for myself. I find it educational to get hands on experience. So, I will post the results after I have run a stereo test (might be a few weeks). Not that I don't trust the experience and knowledge of other people, don't get me wrong

I don't think the objections I've seen expressed here are against indoor use, but rather use in a stereo pair where off-axis response is desired as part of the stereo signal, because the off-axis response varies greatly by angle and frequency. That was or is a somewhat popular method for outdoor recordings where some type of stereo image is being attempted that still rejects the audience, but likely will sound like a phasey mess.

That might very well be the case, sounds entirely plausible. I enjoy doing experiments to test things for myself. I find it educational to get hands on experience. So, I will post the results after I have run a stereo test (might be a few weeks). Not that I don't trust the experience and knowledge of other people, don't get me wrong

Regards, Christine

I'm in the experimental phase myself, and I love it. I have lots of combinations to play with. Some of them defy conventional wisdom but it's fun to hear the results (and I've got some great pulls indoors in acoustically challenging rooms with shotguns). Here is an example: https://archive.org/details/dso2015-04-01.cp4.flac16

I'm in the experimental phase myself, and I love it. I have lots of combinations to play with. Some of them defy conventional wisdom but it's fun to hear the results (and I've got some great pulls indoors in acoustically challenging rooms with shotguns). Here is an example: https://archive.org/details/dso2015-04-01.cp4.flac16

How did you have you Nak300/CP4's setup for that one? Sounds good to me

I don't think the objections I've seen expressed here are against indoor use, but rather use in a stereo pair where off-axis response is desired as part of the stereo signal, because the off-axis response varies greatly by angle and frequency. That was or is a somewhat popular method for outdoor recordings where some type of stereo image is being attempted that still rejects the audience, but likely will sound like a phasey mess.

That might very well be the case, sounds entirely plausible. I enjoy doing experiments to test things for myself. I find it educational to get hands on experience. So, I will post the results after I have run a stereo test (might be a few weeks). Not that I don't trust the experience and knowledge of other people, don't get me wrong

Regards, Christine

I'm in the experimental phase myself, and I love it. I have lots of combinations to play with. Some of them defy conventional wisdom but it's fun to hear the results (and I've got some great pulls indoors in acoustically challenging rooms with shotguns). Here is an example: https://archive.org/details/dso2015-04-01.cp4.flac16

Just recall that it wasn't that long ago that this was how a lot of people recorded GD, myself included! My first serious rig was Nak 300s and a D5.

How about stereo shotguns (without ever having tried it I would say 40-60cm seperation depending on distance, and 0 angle) and a side mic mounted on top of one of the two shotguns? Record each to a separate track. That way after the fact you can choose to do it one way or the other way depending on how it sounds.

My source won't be PA'd but a wind band that I need to mic from about 9 meters away.

The main problems with using shotguns for indoor stereo music recording are that:

(1) they have narrow pickup patterns only in the upper midrange and above (i.e. the part of the speech frequency range that controls intelligibility). Below their "crossover" frequency, they're generally supercardioids or the like. The longer the tube, the lower the crossover point--it's a wavelength thing.

(2) in the frequency range that the tube affects, the response for any given angle of arrival can be extremely irregular--peaks and valleys of 6 to 10 dB can readily occur. And the frequency response varies greatly by angle; those 6-to-10 dB peaks and valleys shift up or down in frequency in response to slight changes in the angle of sound arrival.

(3) the overall, summed pickup of reflected sound in a shotgun microphone (i.e. its "diffuse field" response) is very rolled off in the treble. This is only partly compensated (and in an uneven, unnatural sounding way) by the high-frequency peaks that many shotguns have built in to their overall response.

Consider point (2) carefully. It's why boom operators work so hard to keep shotgun mikes aimed precisely at the person speaking. It's not because the voice won't be heard due to the narrowness of the pattern; the pattern isn't all that narrow. Rather, it's because the voice will be heard, but will tend to sound like shit if it's (say) 45 or 60 degrees off-axis. At frontal angles where shotgun microphones don't suppress sound very strongly, they still only sound good within a narrow angular range. (That problem is why some shotgun microphones cost ten times what others cost--and why professionals use short shotguns rather than long shotguns whenever possible.)

When we record live music from a good miking position, direct sound reaches the microphones from a relatively wide angle. But if we're forced to record from near the back of a room, direct sound is coming from only a narrow range of angles, and so the main part of the sound is arriving at random angles (i.e. it is diffuse sound). That's when point (3) above applies. In that situation, a pair of good supercardioids (with a near-constant pickup pattern across the frequency spectrum) will beat the pants off of any shotgun microphones in the world.

Plus you can use supercardioids as a closely-spaced or coincident pair, which can't be done with shotguns; the shotguns' varying pickup patterns in different parts of the frequency range would logically require you to place them at different angles for different frequencies--which is physically impossible.

(2) in the frequency range that the tube affects, the response at any given angle of sound arrival can be extremely irregular--peaks and valleys of 6 to 10 dB can readily occur. And the frequency response varies greatly by angle; those 6-to-10 dB peaks and valleys shift up or down in frequency in response to slight changes in the angle of sound arrival.

If interested, I've a suggestion on a configuration to experiment with-

As discussed above, shotguns do not work well as a stereo pair in standard stereo pair mic'ing configurations that rely on a smooth, well-behaved transition and "hand-off" between the two across the center region of the sound image due to their irregular off-axis response. So instead of trying to use them as a substitute for well-behaved directional mics, consider using one for what they are best at - maximally isolating whatever they are pointed directly at as much as possible, without regards to the quality of pickup of everything else - and use that to supplement a stereo pair of microphones which provide what a pair of shotguns cannot - a smooth and even stereo image with a good sounding ambient pickup.

I'm suggesting a three microphone configuration with a single shotgun in the center pointed directly at the source, similar to the image posted above of the test recording, with the stereo pair routed Left/Right and the shotgun panned to center.

Consider it something of a more optimal division of labor, with each element contributing what it does best and covering for what the other element does not do as well. The smooth spatial output of the stereo pair buries the off-axis issues of the shotgun, and the shotgun improves the reach and center solidity of the stereo pair. To work best, the stereo pair configuration needs to be to modified to accommodate the presence of the shotgun, so don't just add the single forward-pointing shotgun to a typical ORTF or DIN pair of cardioids. The stereo pair should be spaced wider or angled farther apart, or both, to provide sufficient sonic space for the shotgun to solidify and fill the middle of the image. It's something of a close-dance, both supporting and embracing the other, and bit more room is needed to keep the two partners from stepping on each other's toes than if they were simply out there dancing around on their own. The stereo pair, if listened to in isolation, should be on the verge of "too far apart or too wide", and perhaps "too ambient" with a hole in the middle when the center channel with the shotgun is muted.

Record all three mics separately, and adjust the level of the center channel afterwards to dial in the optimal amount of up-front center information in a way somewhat similar to matrixing a SBD feed with a stereo AUD pair. Think in terms of balancing the drier direct sound (primarily from the shotgun) with the wetter ambient stereo sound (the stereo pair). Conceptually it's also sort of like M/S, but instead of a single figure-8 providing the side information you have a spaced pair instead. I strongly suggest recording all three channels separately instead of mixing the three live and recording to 2-channel, because doing so allows for careful adjustment of the center level (and EQ if necessary) afterwards by ear. That center level adjustment is critical, and the ability to dial it in perfectly by ear afterwards is one of the advantages a technique like this can provide.

The interesting question then becomes "what configuration is most appropriate for the stereo pair?", and that is going to depend on the situation. I've not actually done this with a shotgun (I would, but I don't own one), yet I commonly use a single forward facing cardioid, supercardioid, or hypercard this way. Outdoors I do this with a spaced omni pair, and it allows me to space omnis twice as wide as I would do without the single directional mic in the center (6' instead of 3'). Alternately, or in combination, I'll use a pair of supercardioids spaced about 2' apart (similar to the photo above) but pointing directly to the sides 180 degrees apart. Remember that the presence of the microphone in the center requires the stereo pair to be more widely spaced and/or angled, than they would be alone. So 180 degrees apart may seem ridiculous, but if all three mics were cardioids that configuration would be identical to two DIN pairs sharing one microphone along their common edge. Inside this has worked very well for me from optimal locations, but from far back in the room you may not want to use omnis or supercards pointing directly at the side sidewalls. In that case you might try using cards or supers for the stereo pair arranged at a smaller angle and spaced more widely to compensate for the reduced angle between them. The logical extreme of that would be supers fairly wide-spaced but parallel to the shotgun, maximally excluding ambient room sound and focused on sound arriving from the front. You'll probably will want some angle between them however, even if it's somewhat minimal, like in the photo above.

Worth some experimentation I think, and you already have all the functioning mics you'll need to try it, you just need a recorder which can record 3 separate channels.

I love these discussions. Very educational and excellent ideas I will definitely give that shotgun in the middle suggestion a try. Thanks all!

Here is a somewhat related question. If you have a non-coincident stereo pair. Let's say two supers 30 cm apart, whatever useful angle. If you then take the stereo file and run it through an MS plugin (such as Voxengo MSED) and mute the side signal, leaving only the mid signal. Would that result in the same mono file as what a super would have given if there had been one exactly been the two you originally recorded?

What I suppose I am driving at is this. Could you place for example a pair of supers with a slight angle at 60cm, and in between a pair of shotguns at 40 cm or so. If you could then take the mid signal from those two shotguns and add it to the widely spaced supers, would that give the same result as you describe, without actually having a center shotgun? If yes, you would be able to do both at the same time: the single shotgun as a center mic for the wide supers as a known-good, and a stereo shotgun recording for experimental purposes.

^ The classic "mono-incompatibility" issue- which results from mixing too many signals together which have similar, yet not identical phase, introduced from the close but not coincident spacing between the the mics. In the case of the stereo incompatibility of a pair of spaced omnis, two signals is too many. And that hints at why the stereo pair, used in combination with a single shotgun in the center, needs to be spaced and/or angled wider than it would be if were being used as a single pair alone. In this case you are mixing three channels down to two 3>2 rather than 2>1. The additional spacing and/or angle between the stereo pair reduces the "mono-incompatibility" issue which may occur on each side, between the summed L and C and the summed C and R.

FYI, the isolated Mid channel output from a MS plugin is the same as summing (mixing) the two channels together.

Christine,I am adding to this discussion with my direct experiences using my AKG CK8 short shotguns. I have used them for many years in outdoor situations like us old skool GD tapers used to use the NAk guns. However, I typically use a technique taught me by one of those Old skool NAk tapers. He used to and I mostly point the mics at 0 degrees spread about 20 cm apart and make sure they were not higher than 7 feet, shoulder height was his optimal positioning from the TS at a GD show. This seems to work great for outdoor recordings with large PA's which are somewhat linear arrayed. ( I am recording loud Rock n Roll or semi-loud jambands [most of which have high quality line arrays for PA's] in an open taping environment)However, I have used them in indoor situations as a "stereo pair", attempting to aim them at 0 degrees spaced apart, but often they get aimed at about 110 degree angle in a quasi nortf configuration between them. Sometimes I do the 'PAS" configuration where I carefully aim the mic toward the middle of the line array taking care not to aim that at the bass stacks. I can second some statements from dsatz and gutbucket that angled away from 0 degrees they usually do not produce a great recording. Occasionally I have made some decent recordings with them indoors. one of note is the DBB|Kung Fu show from the Capital Theater Portchester NY which we attempted because previous experiences there resulted in poor cardiod recordings from the SBD area.

NOw, the AKG Ck8 is an upper-mid quality microphone capsule, but is still an interference tube design, so they are very directional, but somehow not as "bassy" as NAK 100/300s.

The gutbucket technique is one I would try given your available mics. kindms and I have been using his AKG 414's to do MS with a cardiod/subcardiod middle and figure 8 side. He has suggested we use a shotgun for the middle which we may do next time we get to do some recording.Good luck with your experiments, and I must say I am liking that Sennheiser MKH805 shotgun mic you are using

I'm calling this one the "Gutbucket array". Pretty much as described above in Gutbucket's post, a pair of supercards, at 60cm with a 40 degree angle and an MKH805 in the middle. OK, it wasn't exactly in the middle but slightly to the left, but I don't think the difference is significant in this case. All three channels mixed equally and brought up to -16 LUFS.

The jet engine in background is the ventilation system.

I kinda like what the MKH805 adds, sound wise. To me it sounds much... "warmer" (an inaccurate term, I know) than just the supercards by themselves.